When synchronous data is transmitted over a channel that is subject to so-called phase hits, the receiver may recover from a phase hit in such a way that the data symbols thereafter received are rotated from their original positions in the signal constellation. In order to recover the transmitted data correctly in the event of such rotations, it is conventional to differentially encode the data, which means that each data word to be transmitted is represented not by a particular data symbol, but rather, by the angular difference in the signal constellation between two successive transmitted symbols. Since a rotation affects all symbols equally, the relative positions in the signal constellation of successive received symbols remains the same, and the transmitted data can still be correctly recovered.
When the channel coder implements a so-called block code, the desired differential encoding can be achieved by, for example, processing in an appropriate way those bits in each channel coder input word whose values determine the quadrant within the signal constellation of the transmitted symbol. Such an approach will not work, however, if other types of channel codes, such as certain so-called trellis codes, are used.